1. Field of Invention
The present invention relates to an improved reinforced coalescing filter tube which may be used in virtually any coalescing filter assembly, and more particularly relates to a coalescing filter tube having a layer of nonwoven material interposed between a vacuum formed filter layer and a reinforcing structure to prevent expansion of the vacuum formed filter layer into the openings in the support structure during use, a situation which has been found to be undersireable because the packing density and, therefore, the pore size of the filter at the openings in conventional constructions has been found to be larger than the packing density and pore size elsewhere in the filter. In addition, the providing of the layer of nonwoven material gives added support in high differential pressure situations.
2. Description of the Prior Art
Most tubular filters of the type with which the present invention is concerned are made for use in filter housings, and are used to flow either in-to-out or, from the outside in. While it is advantageous to flow from the outside in for many filter applications, there is also a definite advantage for flowing in-to-out for certain applications. For example, the coalescing of liquid droplets and aerosols from gases, or the coalescing of two liquid phases. In these applications, it is desireable to have an external support structure to support the filter media and thereby prevent media rupture caused by high differential pressure across the filter. Such external support structures are usually made of metal or plastic.
Filter tubes of this type are commonly manufactured by applying a vacuum to the inside of a porous mandrel and submerging the mandrel in a slurry of fibers of various compositions. The composition of the slurry determines the pore size of the filter. Because of the vacuum applied to the mandrel, the fibers in the slurry are deposited on the surface of the mandrel, the mandrel is then removed from the slurry, and after any free or excess water is removed, a filter tube is left on the mandrel.
The inside diameter of the filter tube is very consistant from filter tube to filter tube because the inside diameter of the tube is a function of the outside diameter of the mandrel. It, additionally, is very smooth and uniform in appearance because it is formed against the mandrel.
However, the outside diameter not only is not uniform, but is very rough in appearance because it does not have any similar structure to form against. This has produced a serious problem in the art of how to apply a support structure to the outside of the tube, and have it in contact with said tube at all points, so that the filter tube does not rupture when pressure is applied thereto.
In order to prevent the filter tube from bursting, it is essential that the filter media is supported by an external support structure in relatively intimate contact with the media. The optimum situation would be to have a support structure in contact with the filter media at an infinite number of points around the outside diameter of the filter tube. However, previous attempts in providing outer support structures have not been entirely satisfactory.
Basically, four methods have been tried. The first method involved placing an outer support structure loosely over the filter media. However, this method does not provide close intimate contact between the filter media and the support core. Thus, rupture of the filter media was very likely to occur at even low differential pressures across the filter media in the neighborhood of 10 to 25 PSID.
The second method involved compressing the filter media between an inner rigid support core and an outer rigid support. This method requires both an internal and an external support structure to be utilized, with said outer support structure having a clapping means for maintaining its position relative to the inner support structure, and for maintaining compression of the filter media. An example of this method can be seen in the U.S. Pat. No. 3,460,680 to Domnick. This method usually required stainless steel support structures for prevention of corrosion, and while it is used to the present day, it is very costly to manufacture, and still does not permit total contact between the filter media and the support structure. For this reason, it is still not satisfactory for many applications.
A third method, involving placing a rigid support structure over the tube without any outward force applied from the inside of the filter tube was tried. However, this method suffers from two deficiencies. There is (1) a lack of intimate contact between the filter media and the support structure, or (2) if the support structure is too small in relation to the outer diameter of the filter tube, there is damage to the filter media while attempting to slip on a support structure. Thus, this method still leaves a serious problem in the prior art.
The latest attempt at solving these problems in the art involves a fourth method where the filter media is brought in intimate contact with a rigid outer support core as a result of an outwardly directed force having been supplied to the internal surface of the filter media during the manufacturing process. One embodiment of this method can be seen in the U.S. Pat. No. 4,052,316 to Berger et. al. This method utilizes a continuous rigid support structure, such as a plastic or metal perforated core, which is slipped over the filter media while the media is still under vacuum on a mandrel. After the tube has been slipped over the media, the vacuum is released, and an outward pressure of air forces the media into intimate interlocking contact with the outer support structure. It actually forces the media into the openings of the support core for an interlocking contact.
The pore size and structure of a filter media is a function of the relative surface area of that filter media which, in turn, is a function of the median fiber diameter and packing density. In those areas where the filter media has been forced into the openings of the outer support structure, the thickness of the media will be greater than the adjacent media which is in contact with the support structure (see FIG. 2). Hence, the packing density of the filter media in the opening will be less than the packing density of the filter media in contact with the support structure, and this creates a lack of uniformity in the filter media with regard to pore size and structure.
Additionally, there is a lack of support of the filter media in the openings of the outer support structure. Therefore, when higher differential pressures are applied from the inside to the outside of the filter media, distortion, or even rupture, of the filter media will occur sooner than in supported areas.